Measurements of exclusive photoproduction processes at large values oftandufrom 4 to 7.5 GeV

Abstract

Exclusive photoproduction cross sections have been measured for the processes y p + r + n , y p -r a p , y p i r r -~+ + , y p i p o P , y p -' K + A , and y p i K + z O at large t and u values at several energies for each process between 4 and 7.5 GeV. These measurements taken together with past data taken at small values of t and u provide complete angular distributions. The data show the usual small t and u peaks and a central region in which the cross section decreases approximately as s -'. The results are discussed within the context of parton or constituent models.

Scaling and resonances in elementaryK+Λphotoproduction

“…[41] as well as exclusive photoproduction of (ρ + ω) mesons [9] which agrees reasonably well with the s −8 scaling of the differential cross section.…”

“…The largest kaon production angles show a uniform rise in the scaled cross section, which we presume is evidence of u-channel contributions. The rise is less pronounced than what was observed in the πN channels, but similar to the π∆ channels [9]. In comparison to previous data from SLAC [9] at W = 2.9 GeV and 3.5 GeV, we see that the agreement is excellent; of course the recent CLAS data have extended the precision and scope of angle and energy coverage greatly.…”

“…In this case the contribution from ψ hadron is suppressed since the dominating vector meson + N → pseudoscalar meson + N rescattering will proceed through the double helicity flip scattering which one expects to be suppressed in the high momentum transfer limit. This expectation is confirmed in the exclusive photoproduction of π-mesons in γN → πN reactions which clearly shows a s −7 scaling starting already at s ≈ 7 − 8 GeV 2 [9,13,14].…”

“…The precision of the previous data [9] on exclusive kaon photoproduction was not sufficient (s −N , N = 7.3 ± 0.4) to rule out unambiguously the contribution from the hadronic component of the photon wave function, which results in N = 8. Figure 1 shows the complete set of differential cross sections dσ/dt from Ref.…”

“…In such models, however, the power law of the energy dependence is rather an accidental result. This situation can not explain the "persistent" consistency of the constituent scaling law observed for many hard exclusive reactions including hadronic [8] and photoproduction [9] reactions, involving proton, deuteron and even 3 He targets at |t|, |u| > M 2 i kinematics (see e.g. Refs.…”